1. Field of the Invention
The present invention relates to a lamp unit for a vehicle headlamp, and more particularly to a projector type lamp unit in which a light-emitting device, such as a light-emitting diode, is used as a light source.
2. Background Art
Recently, a lamp unit in which a light-emitting device, such as a light-emitting diode used as a light source, has been employed in a vehicle headlamp,.
For example, JP-A-2003-317513 (“JP '513”) discloses projector type lamp unit including: a projection lens which is placed on an optical axis extending in a longitudinal direction of a vehicle; a light-emitting device which faces upward and is placed rearward of a rear focal point of the projection lens in proximity to the optical axis; and a reflector which is placed so as to cover the light-emitting device from the upper side, and which reflects light from the light-emitting device forward while shifting the light toward the optical axis.
In the lamp unit disclosed in JP '513, a mirror member having an upward reflecting surface, which extends rearward in parallel with the optical axis from a vicinity of the rear focal point of the projection lens, is disposed between the reflector and the projection lens. Part of reflected light from the reflector is reflected upward by the mirror member, thereby forming a low-beam light distribution pattern in which an upper end portion has a cut-off line as an inverted projection image of a front end edge of the upward reflecting surface.
In contrast, JP-A-2000-348508 (“JP '508”) discloses a projector type lamp unit which does not use a light-emitting device as a light source and includes a movable mirror member. The mirror member can be swung downward by a predetermined angle about a swing axis that extends in a rear end portion of the mirror member in the vehicle width direction.
When a projector type lamp unit having a mirror member such as that disclosed in JP '513 is employed, a low-beam light distribution pattern having a clear cut-off line in an upper end portion can be formed while improving the flux utilization rate of light from a light emitting device. However, in order to form a high-beam light distribution pattern, an additional lamp unit is required.
In a configuration where a movable mirror member such as disclosed in JP '508 is employed as the mirror member and the mirror member is swung downward by a predetermined angle, it is possible to form a high-beam light distribution pattern while the illumination range is expanded to the side above the cut-off line of the low-beam light distribution pattern. Therefore, beam switching between a low beam and a high beam is enabled.
However, in the high-beam light distribution pattern formed by the lamp unit disclosed in JP '508, the lower half portion is formed while maintaining the shape and luminous distribution of the low-beam light distribution pattern. Therefore, for a high-beam light distribution pattern, a short-distance range of a road surface in front of a vehicle is excessively illuminated. There arises a problem in that the visibility of the long-distance range of a road surface in front of a vehicle is not ensured.
The invention has been conducted in view of such circumstances.